1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus or an ink-jet image forming apparatus, and relates in particular to an image forming apparatus that can form an image having a stable quality.
2. Related Background Art
As one problem that is encountered when an electrophotographic image forming apparatus is employed, the quality (density, chromaticity, etc.) of an image formed on a recording medium will vary because of a change in the environment in which the apparatus is employed or because when an apparatus has been in use for an extended period of time, the operation of its individual sections may fluctuate. Especially when an image forming apparatus that forms a color image on a recording medium is employed, since the color balance of an image formed on a recording medium can be destroyed even by a slight change in the environment, or by fluctuations in the operation of the individual sections of the apparatus, it is of course preferable that a constant image quality be maintained, even when the environment is or the operation of the individual sections of the apparatus fluctuate.
Therefore, for an electrophotographic image forming apparatus, correction means are provided for selectively changing several image forming conditions that are related to the amount of exposure, and a developing bias in accordance with the environmental state, such as the absolute humidity, or gradation correction means, such as a lookup table (LUT), for correcting image data in order to form an image having a desired density. When these means are employed, based on the environmental state, such as the absolute humidity read by a humidity sensor, the image forming apparatus can select an image forming condition or an optimal value for gradation correction.
Further, in order to obtain a constant image quality, even when the individual sections of an color image forming apparatus are changed, a method is employed whereby color toners are used to form a density detection toner patch on an image bearing member, such as an intermediate transfer member or a photosensitive drum, and the density of the unfixed toner patch is detected by an unfixed toner density sensor.
According to this method, the results obtained by the unfixed toner density sensor are fed back for an image forming condition, such as the amount of exposure or the developing bias, and the density is controlled. In this manner, an image can be formed for which the quality is stabilized.
However, although the density of a toner image can be controlled for the density control process, during which the unfixed toner density detection sensor is used to detect the density of an unfixed toner patch that is formed on an image bearing member, changes in the color balance can not be controlled for the image transfer process, during which an image is transferred from the image bearing member to the recording medium and is thereafter fixed to the recording medium. The color balance of a toner image is changed by the relative efficiency of the procedure used to transfer the image, and by the heat and the pressure applied to fix the image to the recording medium. Thus, changes in the color balance of an image can not be appropriately controlled by the density control process for which the unfixed toner density sensor is used.
Therefore, an image forming apparatus has been proposed wherein a sensor (hereinafter referred to as a color sensor) is provided to detect the density of a monochromatic toner image or a full-color image that has been transferred and fixed to a recording medium. According to this image forming apparatus, a color toner patch (hereinafter referred to as a patch) for controlling density or chromaticity is formed on the recording medium, the density or the chromaticity detected by the color sensor is fed back for an image forming condition, such as the amount of exposure, a developing bias or a lookup table (LUT), and the density or the chromaticity of the image that is finally output on the recording medium.
The color sensor employs red (R), green (G) and blue (B) light sources as light emitting elements for detecting density or chromaticity, and employs one light receiving element for detecting the reflected components of the light emitted by the light emitting elements. After light beams have been emitted by the R, G and B light emitting elements, the light receiving element detects the reflected components as R output, G output and B output signals. Then, by using the R output, G output and R output, the CMYK color model can be identified and color densities of the CMYK can be detected. In addition, linear conversion can be employed to mathematically process the R output, G output and B output, or lookup table (LUT) conversion can be used to detect the chromaticity.
Besides the above configuration, the light emitting element and the light receiving element can also be so designed that light reflected by the patch is split using a prism, and that signals corresponding to the individual wavelength components are obtained by a line sensor.
An ink-jet image forming apparatus has the same image quality (density-gradation characteristic, etc.) fluctuation problem as has the electrophotographic image forming apparatus, since the color balance of an image may be altered due to time-transient variations in the amount of ink that is ejected, a change in the environment or manufacturing variances in the production of detachable ink cartridges. Therefore, a method has been proposed whereby a color sensor is provided for detecting the surface of a recording medium on which an ink image is formed, and whereby the density or the chromaticity of a patch on the recording medium, which is detected by the color sensor, is employed to control the image forming apparatus and to ensure that an image having a stable quality is formed.
In the above described image forming apparatus, by using a color matching table, RGB data for an image to be output are converted into device RGB signals that are handled by each image forming apparatus, and by using a color separation table, the device RGB signals are converted into YMCK signals that match toner and ink colors. Then, by using a calibration table, the YMCK signals are converted into Y′M′C′K′ signals obtained by correcting the density-gradation characteristic inherent to each image forming apparatus. As a result, therefore, several methods for controlling the density or the chromaticity can be employed.
For example, the calibration table may be corrected using the measured density or the color matching table or the color separation table may be corrected using the measured chromaticity, which provides for the formation of an image having the desired color reproducibility.
However, for a conventional image forming apparatus that detects the chromaticity and the density of the patch formed on a recording medium in order to obtain an image for which the quality is stable, conveying parts, such as a roller, a rolling member and a rib, are present along a conveying path, leading to the color sensor, originating at the image forming unit for forming an image on a recording medium. Thus, the following problems have arisen.
Since micro stripes and roughness of the paper fiber occur when a conveying part rubs against the surface of a toner patch, the colors of the toner patch are mixed or their brightness is changed, and the spectral reflectance factor of light reflected by the toner patch can not be correctly measured by the color sensor.
Since it is especially preferable for the electrophotographic image forming apparatus that, in order to avoid the adverse influence of heat of a fixing device, the color sensor be located at a distance from the fixing device and within a range wherein the switch batch function of a double-sided printing unit can be effectively utilized, the portion of the conveying path following the transferring and fixing process must be extended, and the occurrence of adverse affects attributable to such conveying parts as a roller, a rolling member and a rib is increased.
Further, in an electrophotographic image forming apparatus, unfixed toner, i.e., toner remaining after an image has been fixed by the fixing device, is charged by friction produced between a recording medium and a conveying part and adheres to the conveying part, and a toner stain may occur on the surface of a recording medium that contacts the conveying part.